metricsd: Use product id instead of build target id.

We decided to change the name of this identifier to "product id", the
system property where it is stored will also be named
ro.product.product_id.

BUG: 24302729

Change-Id: I0d2f0867b364de98367f0072d216acec76401c33
diff --git a/metricsd/constants.h b/metricsd/constants.h
index 717e5d2..8a00fc4 100644
--- a/metricsd/constants.h
+++ b/metricsd/constants.h
@@ -28,7 +28,7 @@
 static const char kDefaultVersion[] = "0.0.0.0";
 
 // System properties used.
-static const char kBuildTargetIdProperty[] = "ro.product.build_target_id";
+static const char kProductIdProperty[] = "ro.product.product_id";
 static const char kChannelProperty[] = "ro.product.channel";
 static const char kProductVersionProperty[] = "ro.product.version";
 }  // namespace metrics